Investigation of patinas formed on Chinese bronzes using modern multianalytical techniques

Spectroscopic analysis of corrosion products in a bronze cauldron from the Late Iberian Iron Age

Selected bronze fragments unearthed at Cerro de la Cruz (Almedinilla, southern Spain) were analyzed to determine the chemical composition of the corrosion products formed on their surface. The fragments came from a large bronze cauldron used in an Iberian village that was devastated in the mid II century BCE - possibly around 141 BCE. The fragments were analyzed by using various instrumental techniques including electron scanning microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDS), X-ray fluorescence (XRF) spectroscopy, and also by X-ray diffraction (XRD) and micro-Raman spectroscopy. Based on the results, being buried for a long time caused the main elements in the alloy to mineralize and form stratified layers consisting of various mineral phases including cuprite (Cu₂O), malachite [Cu₂CO₃(OH)₂] and cassiterite (Sn₂O). The fragments also contained chloride and trihydroxychloride compounds such as nantokite (CuCl) and atacamite [Cu₂Cl(OH)₃], respectively, which make conservation of archaeological objects troublesome. These results testify to a strong interaction of the alloy elements with soil components. Also, the results obtained suggest a Type I of corrosion structure. Using the SEM-EDS, XRD and XRF and micro-Raman spectroscopies allowed corrosion products in the fragments to be successfully characterized in microchemical and structural.